I've been led to believe there's a difference between how a MacBook and an iPhone/iPad consume power: when a MacBook with a fully charged battery is plugged in, its components draw power directly from the charger while when an iPhone/iPad with a fully charged battery is plugged in, its components still draw power from the battery which is frequently recharged to keep it at 100%. Hence, it doesn't make a difference to the battery's overall lifespan whether the iPhone/iPad is plugged in while you use the device.

Or in other words, if I'm using an iPhone/iPad and am not worried about battery life, there's no point in plugging it in anyway in the hope of prolonging battery lifespan?

3 Answers
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Deeper discharge/charge cycles shorten an Li-Ion battery's lifespan faster that shallow ones. (e.g. One 100% cycle is more wearing that four 25% cycles, etc.) Therefore, keeping an iPad or iPhone on a charger or always topped off between short uses will tend to lengthen the battery's total lifespan by reducing the number of deeper cycles per annum, all other thing being equal (such as total time of use).

"Calibrating" has almost nothing to do with the battery lifespan. It's mostly needed to help the battery gauge icon behave more accurately. Once or twice a year is probably good enough for that purpose.

Agree with hotpaw2's answer, and I'll add. Note that heat is an enemy of electronics. Have 2 friends whose iPhone were damaged from being plugged-in whenever they're not moving around.
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Global nomadMay 27 '12 at 11:47

@Globalnomad: so you're saying that while it may be better for the battery lifespan to keep the iPhone/iPad plugged in if possible, it's also damaging to the rest of the device because of the heat it generates? So I'm not sure now whether it's better to keep it plugged in if possible or not.
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RinzwindMay 27 '12 at 17:48

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Rinzwind, what I'm saying is if you're gonna/have to charge for prolonged time, keep the device in a cooler environment (air conditioned, away from sunlight, heat sources).
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Global nomadMay 27 '12 at 18:02

As far as I know, most mobile phones including iphone in the market will have their battery life shortened if you constantly use them plugged in.

The power keeps going charger - battery - phone.

Since this batteries have what some call "Memory". If you have them constantly plugged in, it will affect that "memory". On a more technical term is like with old PC laptops, since the power is still going through the battery, the battery cell contacts get over used because of the constant power.

The best thing is once a month or every two months calibrate the battery like with laptops. Empty it completely, then plug it in and leave it charging without turning on your device. This will improve the battery life in the long run.